Glucose Stimulates Ca2+ Influx and Insulin Secretion in 2-Week-old β-Cells Lacking ATP-sensitive K+ Channels

Andras Szollosi,Myriam Nenquin,Lydia Aguilar-Bryan,Joseph Bryan,Jean-Claude Henquin

doi:10.1074/jbc.m609875200

Andras Szollosi, Myriam Nenquin + Show 3 more

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https://doi.org/10.1074/jbc.m609875200

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Abstract

In adult beta-cells glucose-induced insulin secretion involves two mechanisms (a) a K(ATP) channel-dependent Ca(2+) influx and rise of cytosolic [Ca(2+)](c) and (b) a K(ATP) channel-independent amplification of secretion without further increase of [Ca(2+)](c). Mice lacking the high affinity sulfonylurea receptor (Sur1KO), and thus K(ATP) channels, have been developed as a model of congenital hyperinsulinism. Here, we compared [Ca(2+)](c) and insulin secretion in overnight cultured islets from 2-week-old normal and Sur1KO mice. Control islets proved functionally mature: the magnitude and biphasic kinetics of [Ca(2+)](c) and insulin secretion changes induced by glucose, and operation of the amplifying pathway, were similar to adult islets. Sur1KO islets perifused with 1 mm glucose showed elevation of both basal [Ca(2+)](c) and insulin secretion. Stimulation with 15 mm glucose produced a transient drop of [Ca(2+)](c) followed by an overshoot and a sustained elevation, accompanied by a monophasic, 6-fold increase in insulin secretion. Glucose also increased insulin secretion when [Ca(2+)](c) was clamped by KCl. When Sur1KO islets were cultured in 5 instead of 10 mm glucose, [Ca(2+)](c) and insulin secretion were unexpectedly low in 1 mm glucose and increased following a biphasic time course upon stimulation by 15 mm glucose. This K(ATP) channel-independent first phase [Ca(2+)](c) rise was attributed to a Na(+)-, Cl(-)-, and Na(+)-pump-independent depolarization of beta-cells, leading to Ca(2+) influx through voltage-dependent calcium channels. Glucose indeed depolarized Sur1KO islets under these conditions. It is suggested that unidentified potassium channels are sensitive to glucose and subserve the acute and long-term metabolic control of [Ca(2+)](c) in beta-cells without functional K(ATP) channels.

Highlights

Fine control of insulin secretion by pancreatic ␤-cells is critical for glucose homeostasis: excessive release causes hypoglycemia, whereas insufficient insulin leads to diabetes
Our observations provide a plausible explanation for the lack of hypoglycemia in Sur1KO mice and indicate that KATP channels are not the only metabolic sensor involved in glucose regulation of Ca2ϩ influx in ␤-cells
Mechanism of Glucose-induced [Ca2ϩ]c Rise in Sur1KO Islets—In this series of experiments we investigated the possible mechanisms underlying the biphasic increase in [Ca2ϩ]c induced by 15 mM glucose in ␤-cells lacking KATP channels (Fig. 5A)

Summary

Introduction

Fine control of insulin secretion by pancreatic ␤-cells is critical for glucose homeostasis: excessive release causes hypoglycemia, whereas insufficient insulin leads to diabetes. Glucose activates an amplifying pathway, termed the augmentation or KATP channel-independent pathway, which does not increase [Ca2ϩ]c further but augments the secretory response to the triggering Ca2ϩ signal The mechanisms for this metabolic amplification have not been fully identified but are distinct from neuro-hormonal amplification (1, 9 –13). Mice lacking KATP channels in their ␤-cells (Sur1KO or Kir6.2KO mice) are valuable models to investigate the regulation of insulin secretion without using pharmacological agents. Our aim was to determine whether glucose can control insulin secretion in these young ␤-cells lacking KATP channels and to assess whether such control is exerted via changes in the triggering Ca2ϩ signal or via the amplifying pathway. Our observations provide a plausible explanation for the lack of hypoglycemia in Sur1KO mice and indicate that KATP channels are not the only metabolic sensor involved in glucose regulation of Ca2ϩ influx in ␤-cells

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